Privacy system



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By H. W. DUDLEY A T TORNE lf ,Patente Sept. 3, i941 I rg Aer l gra Peterson, lNew York,

and Homer W. Dudley,

Garden City, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 10, 1938, Serial'No. 229,236

4 Claims.

Thepresent invention relates to systems and methods of transmission with privacy; It is appreciab1eto line Wire or radio transmission, -and to the sending of speech messages or any other 5 type of signalsvwhich comprise a band of frequency components such that the signal Wave band can be subdivided into narrower 'subbanda A general object of the invention is to increase the diiiiculty of unauthorized reception of l the signal or other wave being transmitted.

Prior art systems have been devisedV whicn required for'successful reception the provision by the listener of a-wave of proper frequency, the

selection of particular received frequencies and 35. the rejection of others. Various'means have been suggested to increase the difficulty of hitting upon the right frequency to be used and the band to be selected from incoming waves.

The present invention in its preferred form 20 renders successful reception very difficult without a knowledge of the scheme of wave transformation used, by placing strict" requirements on both the frequency and phase relations which must -be observed for successful reception. 25 In accordance with the inventiofi 'tfemessage Waves are subdivided into narrow bands which are simultaneously transmitted `on a time-division or phase-discrimination basis, similarly to multiplex transmission, wherebythe subbands 3c are separately reproduced at the receiver. They are then combined in proper manner to recon struct the message waves.

The various features and objects of theinvengvtion will be made clear from the following deas tailed description of an illustrative embodiment.

In the drawing,

Fig. 1 is a schematic diagram of the apparatus which may be used at one terminal of the system in accordance with the invention; and

a@ Figs. 2 and 3 are frequency diagrams to be referred to in the description.

Referring rst to Fig. 1, the incoming line L1 from any suitable signal source such as a telephone transmitter is connected in multiple to the g5 subdividing band lters F1., F2, F3, F4 and F5 which subdivide the incoming wave band into (in this case). five subbands, each 500 cycles wide by way of example. The band limits are given on the drawing for illustrative purposes. The output 50 side of each lter is connected to a corresponding modulator l0 to lll, inclusive, each of which is supplied by a wave of appropriate frequency from a wave source l5, i6, etc. to step the frequencies down to the range of zero to 500 cycles. Modu- 55 lator l0, for example, is vsupplied with a frequency of 750 cycles, this being the upper cut-off*` A frequency of the lter F1. This results in the\ production of upper and lower side-bands, the lower side-band extending from zero to 500 cycles. The lter Fs suppresses the upper sideband and may assist the modulator l0 in suppressing the unmodulated carriercomponent. The same sort of action occurs in each of the other modulators. These modulators are preferably of the balanced or carrier suppression type.

Each of the filters F6 to F1o has one terminal common to` one terminal of the outgoing line L2. The other terminal of each filter F6 to F1o is connect'ed to a different one of the five segments of the rotary distributor 2i), the rotating arm of which is connected to the remaining side of the outgoing line L2. The iilter Fn is used in the outgoing line to limit the transmitted band to the range of zero to 2500 cycles and equalizer 2| equalizes the line for constant attenuation over the frequency band and linear phase shift.

The rotating arm of the distributor 20 is driven at a highly constant speed which is at least as great as 1000 revolutions per second or twice as high` as the highest frequency to be sent. The

active segments of the commutator are shown as relatively short leaving relatively long idle spaces between. This is in accordance with the theory and practice disclosed in W. R. Bennett application, Serial No. 221,298, filed July 26, 1938. As disclosed in the Bennett application the use of a proper switching function results-in the production of side-bands on a. succession of carrier waves harmonically related .in frequency and of equal amplitude. When these relations hold, the frequency band transmitted may be limited to the product of the number of channels' by the band width of one channel, and the cross-talk be` tween channels may be made ideally zero. The proper switching function is controlled by the speed of rotation of the distributorA arm and the shape and relative length of the active segments.

The relation of the side-bands and carrier wave components in the system as described in Fig. 1 is illustrated in the diagram of Fig. 2. Five side-bands are present extending from zero to 2500 cycles The carriers in this case have frequencies of zero, 1000 cycles and 2000 cycles. Sinceyno direct current is supplied to the 4distributor, the carrier frequency components are not actually produced; The lowest side-band extends from Zero to 500 cycles and is based upon-a direct current carrier. The 1000 cycle carrier has lower and upper side-bands extending downward to 500 cycles and upward to 1500 cycles and the 2000 cycle carrier wave has upper and lower side-bands extending downward to 1500 cycles and upward to-2500 cycles. (For clarity in illustration a slight gap is shown between the upper sideband and the4 lower side-band of the next higher carrier, although theoretically no gap need exist in these points.) The rotary distributor, of

course, produces side-bands based on higher'frequencies than those illustrated but these are 'removed by the filter F11 which is designed to have a sharp cut-off to suppress frequencies above2500 cycles. vide an infinitely sharp cut-olf so as completely to separate two adjacent sideebands, the lter may advantageously be designed in -accordance with the principles disclosed in Nyquist Patent 1,748,486, February 25, 1930, according `to which I ferent channels. The three carriers shown may be visualized as modulated in zero phase by the signal from filter F6 to produce all of the sidebands illustrated in Fig. 2. 'I'his occurs byy the passage of the rotating arm over the corresponding commutator segment. These wave components, however, persist in time after the brush has passed ofi the segment to which the upper channel is connected. This Will be evident from noting that the band Width transmitted is limited by the filter F11 to a comparatively narrow range. The high frequencies required by the Fourier analysis to extinguish the frequencies resulting from any one channel at all times except when the brush is in contact with the corresponding segment, are not permitted to be transmitted. The side-hand frequencies of one channel are, therefore, present on the line at the same time as frequencies produced by others of the channels. The same frequencies illustrated in Fig. 2 are produced lby passage of the rotating brush over each of the succeeding channel segments but with successive phase displacements.

Reception of the wave isaccomplished by a station at the opposite end of the line L2 Which may be the exact counterpart of that shown in the drawing and which, therefore, has not been illustrated. The action may be understood by considering a wave incoming on line L2 from a distant station, considering Fig. 1 as the receiving station. It is assumed that the rotating brushes at the two stations are in exact synchronism and phase. The passage of the rotating brush over the segment of a particular channel results in the production of a set of Waves in such phase as to reproduce the subband corresponding to that channel by arithmetical addition of the components from all five side-bands (see Fig. 2). At the same time the phase displacement between the waves produced by the receiver distributor and the incoming side-bands representing the other four channels are such that in the ideal case all of the detected or demodulated components of those other channels add to zero. When the brush passes onto the next segment the phase relations are right for reproducing the signal corresponding to that channel and for producing a zero resultant for the signals -belonging to the other channels.

Since it is impossible for a filter to procycles.

The requirement of transmitting a direct cur-v rent component over the line as in the system of Fig. 1 may be avoided by changing certain of the constants of the system as will now be described in connection with Fig. 3. In this case the lters corresponding to Fe to F1o, -inclusive, each pass the band of 250 to 750 cycles and the switching the sources I6 to' I9, inclusive.

As seen from Fig. 3, the carrier vfrequencies in this case would be zero, 1500 cycles and 3000 As in the previous case, however, the carrier frequencies themselves are not transmitted. The lowermost side-band extends from 250 to 750 cycles and the other side-bands are as shown in the diagram. The cut-01T characteristic of filter F11 for this case is indicated at B. Y

The foregoing' description is based upon the use of a. mechanical type of distributor. It is clear, however, that. the only requirement is that of producing sets of side-bands based upon carrier waves of proper frequency and phase relations and any terminal apparatus capable of accomplishing these effects may be used to practice .the invention. An electronic type of rotary disin United States patent application of E. Peter-F son, Serial No. 221,297, filed July 26, 1938, for building up the sets of side-bands based on carriers properly displaced in phase. f

The invention is not to be construed as limited to the structural details nor to the magnitudes given in the foregoing description but its scope is defined in the claims.

What is claimed is:

l. 'Ihe method of transmitting message waves with privacy comprising subdividing the Waves into frequency subbands in different circuits, re-

' ducing the subbands to occupy the same absolute frequency limits, transmitting the resulting subband waves as modulations of thesame set of carrier waves, the modulations by the respective subband waves being displaced in phase and utilizing the phase displacement between the modulated waves to distinguish the several message subband waves from one another.

2. In transmission of message waves-with privacy, the method comprising subdividing the message waves into frequency subbands, shifting the frequency of certain subbands so that all subbands occupy the same frequency limits, producing from one subband, modulations of harmonically related carrier waves in one phase, profducing modulations of the same carrier waves in different phase by another of the subbands and utilizing the phase displacement between the the signal band but shifted to occupy the same absolute frequency subband limits, means to produce from the rst such subband a number of modulation side-bands of different frequency equal to the number of subbands into which the signal waves are subdivided, means to produce a like number of side-bands of the same .respective frequencies in the case of the second subband, and so on, including means to produce equal phase rotations between the side-bands produced from one subband and those produced from the next, and means to transmit the resulting sidebands.

4. A receiving system from signals transmitted in the form of successions of discrete side-bands, each side-band in the same succession representing components taken from one portion of the signal band and repeated over and over again throughout the total frequency range embraced by the side-bands, one succession of side-bands being rotated in phase with respect to another, said system including a plurality of circuit branches and a recelver common thereto, and

comprising demodulating means operating in such frequency and phase as to reproduce in each circuit the signal components represented by a respective succession of side bands with substantial suppression of the components represented ,by other successions of side-bands, and means to combine in said receiver the signal components reproduced in the several circuits.

ROBERT C. MATHES.

EUGENE PETERSON.

HOMER W. DUDLEY. 

